This invention pertains generally to the field of tracking optical telescopes and, in particular, to the filters which select particular optical wavelengths to be tracked on. Specifically, it pertains to the use of an atomic resonance filter to serve this purpose.
Tracking optical telescopes are commonly used to track moving bodies which directly or reflectively emit optical radiation. Frequently, the optical tracker employs an optical filter which selects a particular optical wavelength to track on, rejecting the rest of the optical spectrum. Such an optical filter aids the effectiveness of the tracking telescope by rejecting background light and increasing the signal-to-noise ratio (SNR) of the optical tracking signal. The standard interference optical filters used for selecting a narrow band of optical wavelengths preserve the imaging properties of the telescope. Consequently, these optical filters preserve the incident angle of the optical signal, which manifests itself as a position in the focal plane of the imaging optics. A suitable segmented or multi-array sensor can detect the displacement of the incident angle and emit electrical signals proportional to this displacement. A commonly used electronic servosystem can use these electrical signals to reposition the telescope so that the optical image is centered on the optical signal. Hence, the use of such a sensor permits the telescope to effectively track incoming light signals.
Recently, the atomic resonance filter (ARF), comprised of vapors of specific atoms, has been developed, as a new type of narrow band optical filter. (Cf. "Atomic Resonance Filters", Jerry A. Gelbwachs, IEEE Journal of Quantum Electronics, Vol. 24, No. 7, July 1988, and U.S. Pat. No. 4,829,597). Typically, an incoming signal of a specific wavelength entering the atomic resonance filter elevates the atoms therein into an excited state, which state then deploys in a two-or multi-step cascade, emitting light at different wavelengths. A suitable sensor collects the light signals at the new wavelength. An appropriate arrangement of optical cutoff filters before and after the cell containing the vapors renders this device a very effective narrow band optical filter. Unfortunately, this class of atomic resonance filters loses the imaging properties of the filters and, consequently, atomic resonance filters do not function well in a tracking telescope.